It is known that heat insulating bodies (vacuum panels) can be manufactured from a porous material and a gas-tight sheet which provides an all-round cladding, the "hollow space" formed by the porous material being evacuated. The porous material may consist either of an expanded bulk material, or of a rigid foamed material, however. The sheet usually consists of combinations of several layers, with different functions being performed by the different layers, such as a diffusion barrier to various gases and mechanical reinforcement.
Most commercially available vacuum panels have a core of inorganic bulk material, which is usually silicate-based.
EP-B-188 806 describes rigid foamed materials based on polyurethane (PUR) as a filler material for vacuum panels. The problem with this process is to achieve the requisite open cellularity by the precise selection of the formulation and by adhering to defined process parameters, since PUR rigid foamed materials are predominantly of closed cell structure. The outgassing of volatile constituents such as amines, foaming agents, and possibly monomers also, constitutes another problem when using PUR rigid foamed materials as a support material for vacuum panels.
On the other hand, one advantage of rigid PUR foamed materials compared with porous inorganic bulk materials is their accurate sizing as finished vacuum panels, as well as their dimensional stability.
Surprisingly, it has now been found that an excellent support material for vacuum panels can be produced in an economical manner by producing a porous bulk material by pulverising rigid foamed materials and processing this bulk material--similarly to porous silicate bulk materials--to form vacuum panels. In this connection rigid foamed materials are preferably used which arise in the recycling of valuable substances, and the PUR foamed material which arises on the recycling of refrigerators is particularly preferred in this respect.